As is well-known, a piezoelectric material whose polarization direction is the same as that of the electric field expands in the electric field direction and contracts in the direction perpendicular to the electric field, and a piezoelectric material whose polarization direction is opposite to that of the electric field contracts in the electric field direction and expands in the direction perpendicular to the electric field. Certain ceramic material, for example, PZT, is also known to be a piezoelectric material. A bimorph element comprising two layers of this ceramic material expands and contracts in accordance with the direction of polarization thereof, according to the polarity of an electric field applied thereto. Thus, if a bimorph element is formed of ceramic material with one end fixed to a suitable support structure, the free end moves in one direction or the other in accordance with an applied electric field.
The bending movement of a bimorph element has been used in vibration generator, vibration sensor, and electrical switching devices. In the present invention it is proposed to automatically open or close a balanced Venetian blind through the displacement of the free end of a bimorph element formed of ceramic material, with one end fixed and the element driven by an output of a photoconductive circuit. When the bending motion of a bimorph element causes a window blind to open or close, it is desirable that a large bending movement be obtained in accordance with changes of incoming light quantity and with high light sensitivity. A bimorph element should be driven by a direct current voltage source under the applicable maximum voltage which will not produce polarization inversion causing the bending sensitivity to decrease. It is disadvantageous from the point of material cost to increase a ceramic bimorph element structure in size. There is a need in the art for a photoelectric circuit arrangement which is characterized by a simple electrical arrangement, the use of only one direct current voltage source, direct driving for a bimorph element by the voltage generated between two outputs of photoconductive networks, and the capability of making a piezoelectric bimorph element to maximally bend with high light sensitivity according to the light quantity.